Idle speed controller for internal combustion engine

ABSTRACT

An idle speed controller for an internal combustion engine provided with an intake variable valve timing mechanism for varying valve timing and a variable working angle for varying a valve working angle of an intake valve. The controller performs idle speed control. When an advancement failure occurs in the intake variable valve timing mechanism, the controller performs idle-up control to increase the target idle speed. When the valve working angle of the intake variable working angle mechanism is less than a predetermined value, the electronic control unit changes an increase amount of the target idle speed to a lower value if an advancement failure occurs when performing the idle-up as compared to when the valve working angle is greater than the predetermined value.

BACKGROUND OF THE INVENTION

The present invention relates to an idle speed controller forapplication in an internal combustion engine provided with a variablevalve timing mechanism and a variable working angle mechanism, and moreparticularly, to an improvement of a control structure for preventingmisfires when an angle advancement failure occurs in a variable valvetiming mechanism.

In the prior art, a variable valve timing mechanism is used to vary thevalve timing of an intake valve and an exhaust valve. The variable valvetiming mechanism uses a hydraulic or electric actuator to alter therotation angle of a camshaft relative to a crankshaft in order to varythe valve timing of an intake valve and an exhaust valve. In an internalcombustion engine that uses such a variable valve timing mechanism forits intake system, misfires may occur when the engine is idling. Suchmisfires occur when the valve timing of the intake valve is advancedfrom the intended timing. Such a state is referred to as an advancementfailure of the variable valve timing mechanism. The intake air amount issmall when the engine is idling. If an advancement failure occurs insuch a state, overlap between the periods in which the intake andexhaust valves are both open (hereafter, referred to as “valve overlap”)increases. This increases internal exhaust gas recirculation (EGR) anddeteriorates combustion conditions.

To solve this problem, Japanese Patent Laid-Open Publication 2003-65131proposes a technique for setting a target idle speed to a value higherthan normal during execution of idle speed control when an advancementfailure occurs in the variable valve timing mechanism. Such a techniqueis referred to as “idle-up control”. The engine speed is increased whenthe engine is idling to increase the intake air amount. This lowers theinfluence that an increase in internal EGR caused by an advancementfailure has on combustion. This relieves the effect on the combustionconditions by the increase in the internal EGR caused by an advancementfailure. Accordingly, idle-up control prevents misfires that would becaused by an advancement failure in the variable valve timing mechanism.

Japanese Patent Laid-Open Publication 2001-263015 describes a variableworking angle mechanism that varies the valve working angle of intakeand exhaust valves (range of crank angle from when the valve opens towhen the valve closes). In an internal combustion engine that uses suchvariable working angle mechanism for its intake system, the amount ofair drawn into a cylinder is adjusted not only by varying the throttleopen amount but also by varying the valve working angle of the intakevalve. Therefore, the required amount of air drawn into the cylinder isensured through the combination of the throttle open amount and thevalve working angle. For example, when the engine is operating at apredetermined throttle open amount and valve working angle, the throttleopen amount may be increased or decreased. In such a case, the amount ofair drawn into a cylinder is kept constant by decreasing or increasingthe valve working angle of the intake valve so as to decrease orincrease the drawn in air amount by the amount of air that is increasedor decreased by the increase or decrease of the throttle open amount.Even though the air amount is kept constant, a fluctuation may occur inthe amount of pumping loss or compression end pressure and compressionend temperature (the pressure and temperature in a cylinder at thecompression top dead center) depending on the combination of thethrottle open amount and the valve working angle. Therefore, in aninternal combustion engine using such a variable working angle mechanismfor its intake system, the combination of the throttle open amount andthe valve working angle is changed if necessary even when the air amountis constant. This enables the air amount to be controlled so that theinternal combustion engine is operated under further favorableconditions.

In an internal combustion engine that executes air amount control insuch a manner, the valve working angle of the intake valve is not alwaysconstant even when the engine is idling. Thus, depending on the settingof the valve working angle, when an advancement failure occurs in thevariable valve timing mechanism, the increase amount of the internal EGRrelative to an increase of valve overlap may change. In such an internalcombustion engine, if the idle-up control is performed in the samemanner whenever an advancement failure occurs, the idle-up amount maybecome excessive or insufficient. As a result, execution of unnecessaryidle-up control may lower the fuel efficiency when the idle-up amount isexcessive or cause misfires when the idle-up amount is insufficient.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an idle speedcontroller for an internal combustion engine provided with a variablevalve timing mechanism and a variable working angle mechanism capable ofefficiently and effectively preventing the occurrence of misfires in theinternal combustion engine when a failure occurs in the variable valvetiming mechanism.

One aspect of the present invention is an idle speed controller forexecuting idle speed control for an internal combustion engine includingan intake valve and an exhaust valve. The idle speed controller includeseither one of an intake variable working angle mechanism for variablysetting an working angle of the intake valve and an exhaust variableworking angle mechanism for variably setting an working angle of theexhaust valve. A variable valve timing mechanism variably sets valvetiming for the intake valve. An electronic control unit executes idlespeed control. When the internal combustion engine is idling and anadvancement failure of the variable valve timing mechanism occursthereby advancing the valve timing of the intake valve from an intendedtiming, the electronic control unit performs idle-up during execution ofthe idle speed control to increase a target idle speed. When the workingangle set by either one of the intake and exhaust variable working anglemechanism is less than a predetermined value, the electronic controlunit changes an increase amount of the target idle speed to a lowervalue if an advancement failure occurs when performing the idle-up ascompared to when the working angle is greater than the predeterminedvalue.

A further aspect of the present invention is an idle speed controllerfor executing idle speed control for an internal combustion engineincluding an intake valve and an exhaust valve. The idle speedcontroller includes either one of an intake variable working anglemechanism for variably setting an working angle of the intake valve andan exhaust variable working angle mechanism for variably setting anworking angle of the exhaust valve. A variable valve timing mechanismvariably sets valve timing of the intake valve. An electronic controlunit executes idle speed control. When the engine is idling and anadvancement failure of the variable valve timing mechanism occursthereby advancing the valve timing of the intake valve from an intendedtiming, the electronic control unit performs idle-up during execution ofthe idle speed control to increase a target idle speed. When the workingangle set by either one of the intake and exhaust variable working anglemechanism is less than a predetermined value, the electronic controlunit prohibits the idle-up if an advancement failure occurs.

Another aspect of the present invention is an idle speed controller forexecuting idle speed control for an internal combustion engine includingan intake valve and an exhaust valve. The idle speed controller includeseither one of an intake variable working angle mechanism for variablysetting an working angle of the intake valve and an exhaust variableworking angle mechanism for variably setting an working angle of theexhaust valve. A variable valve timing mechanism variably sets valvetiming of the intake valve. An electronic control unit for executingidle speed control. When the internal combustion engine is idling and aretardation failure of the variable valve timing mechanism occursthereby retarding the valve timing of the intake valve from an intendedtiming, the electronic control unit performs idle-up during execution ofthe idle speed control to increase a target idle speed. When the workingangle set by either one of the intake and exhaust variable working anglemechanism is less than a predetermined value, the electronic controlunit changes an increase amount of the target idle speed to a lowervalue if a retardation failure occurs when performing the idle-up ascompared to when the working angle is greater than the predeterminedvalue.

A further aspect of the present invention is an idle speed controllerfor executing idle speed control for an internal combustion engineincluding an intake valve and an exhaust valve. The idle speedcontroller includes either one of an intake variable working anglemechanism for variably setting an working angle of the intake valve andan exhaust variable working angle mechanism for variably setting anworking angle of the exhaust valve. A variable valve timing mechanismvariably sets valve timing of the intake valve. An electronic controlunit executes idle speed control. When the engine is idling and aretardation failure of the variable valve timing mechanism occursthereby retarding the valve timing of the intake valve from an intendedtiming, the electronic control unit performs idle-up during execution ofthe idle speed control to increase a target idle speed. When the workingangle set by either one of the intake and exhaust variable working anglemechanism is less than a predetermined value, the electronic controlunit prohibits the idle-up if an advancement failure occurs.

Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic diagram showing a preferred embodiment of thepresent invention;

FIG. 2 is a graph showing changes in the valve timing of an intake valvewhen an intake variable valve timing mechanism is actuated;

FIG. 3 is a graph showing changes in the valve working angle of theintake valve when a variable working angle mechanism is actuated;

FIG. 4 is a graph showing the relationship between the valve workingangle of the intake valve and a valve overlap amount;

FIG. 5 is a flowchart showing the procedures for setting a lower limitvalue;

FIG. 6 is a schematic diagram showing a map A that is used to set thelower limit value;

FIG. 7 is a schematic diagram showing a map B that is used to set thelower limit value; and

FIG. 8 is a flowchart showing the procedures for setting the lower limitvalue.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be describedwith reference to the drawings.

As shown in FIG. 1, an internal combustion engine 10 includes an intakepassage 12 provided with a throttle valve 14. The throttle valve 14 isconnected to a throttle motor 16. The throttle motor 16 is driven toadjust the amount the throttle valve 14 is open (throttle open amountTA) and control the amount of air drawn into a combustion chamber 18through the intake passage 12. The intake passage 12 further includes aninjector 20, which injects fuel into the intake passage 12.

Air-fuel mixture formed by intake air and injected fuel is ignited by anignition plug 22 in the combustion chamber 18 of the internal combustionengine 10. The ignition timing of the ignition plug 22 is adjusted by anigniter 22 a. The ignition burns the air-fuel mixture and reciprocates apiston 24 so as to rotate a crankshaft 26. Subsequent to combustion, theair-fuel mixture is discharged as exhaust from the combustion chamber 18and into an exhaust passage 28.

In the internal combustion engine 10, the intake passage 12 and thecombustion chamber 18 are connected to and disconnected from each otherby opening and closing of an intake valve 30. The combustion chamber 18and the exhaust passage 28 are connected to and disconnected from eachother by opening and closing of an exhaust valve 32. Rotation of anintake camshaft 34 opens and closes the intake valve 30. Rotation of anexhaust camshaft 36 opens and closes the exhaust valve 32. Rotation ofthe crankshaft 26 rotates the intake camshaft 34 and the exhaustcamshaft 36.

The intake camshaft 34 is provided with an intake variable valve timingmechanism 38. The intake variable valve timing mechanism 38 adjusts therotation angle of the intake camshaft 34 relative to the rotation angleof the crankshaft 26 (crank angle) so as to advance or retard theopening or closing timing (valve timing) of the intake valve 30. Theintake variable valve timing mechanism 38 is actuated by controlling thehydraulic pressure applied to the mechanism 38 by an actuator 40 such asa hydraulic control valve. FIG. 2 shows changes in the valve timing ofthe intake valve 30 when the intake variable valve timing mechanism 38is actuated. As shown in FIG. 2, when the valve timing is varied, theopening timing and the closing timing of the intake valve 30 are bothadvanced or retarded in a state in which the valve working angle of theintake valve 30 (the range of the crank angle from opening to closing ofthe valve) is kept constant.

The exhaust camshaft 36 is provided with an exhaust variable valvetiming mechanism 42. The exhaust variable valve timing mechanism 42adjusts the rotation angle of the exhaust camshaft 36 relative to thecrank angle so as to advance or retard the valve timing of the exhaustvalve 32. The exhaust variable valve timing mechanism 42 is actuated bycontrolling the hydraulic pressure applied to the mechanism 42 by anactuator 44 such as a hydraulic control valve. In the same manner as inthe intake valve 30, the opening timing and the closing timing of theexhaust valve 32 are both advanced or retarded in a state in which thevalve working angle of the exhaust valve 32 is kept constant. In thisembodiment, the exhaust variable valve timing mechanism 42 functions asa variable exhaust valve closing timing mechanism for varying theclosing timing of the exhaust valve.

An intake variable working angle mechanism (hereafter referred to as“variable working angle mechanism”) 46 is arranged between the intakecamshaft 34 and the intake valve 30. The variable working anglemechanism 46 variably sets the valve working angle of the intake valve30 in accordance with operation conditions of the engine. The variableworking angle mechanism 46 is actuated by driving an actuator 48 such asan electric motor. FIG. 3 shows changes in the valve working angle ofthe intake valve 30 caused by actuation of the variable working anglemechanism 46. As shown in FIG. 3, the actuation of the variable workingangle mechanism 46 varies the valve working angle of the intake valve 30in synchronization with the lift amount. For example, as the valveworking angle decreases, the lift amount decreases. When the valveworking angle is increased, the interval between the opening timing andthe closing timing of the intake valve 30 increases. This lengthens theopen period of the intake valve 30.

In this embodiment, the intake air amount is adjusted by cooperationbetween control of the open amount of the throttle valve 14 (throttlecontrol) (see FIG. 1) and control of actuation of the variable workingangle mechanism 46 (variable valve working angle control). The intakeair amount increases when the throttle open amount TA increases and whenthe valve working angle of the intake valve 30 increases. Therefore,during the cooperative control, the throttle control and the variablevalve working angle control are respectively executed such that thethrottle open amount TA is set to be relatively small when increasingthe valve working angle of the intake valve 30, and the throttle openamount TA is set to be relatively large when decreasing the valveworking angle of the intake valve 30. This adjusts the intake air amountin a desired manner. The valve working angle of the intake valve 30 isadjusted by the cooperative control so that the valve working angle issmaller after the internal combustion engine 10 is warmed than before itis warmed. More specifically, the valve working angle of the intakevalve 30 is adjusted so that it is smaller when the temperature ofengine coolant is higher.

The idle speed controller of this embodiment has various sensors fordetecting operation conditions of the internal combustion engine 10.These sensors include, for example, a crank sensor for detecting thespeed of the crankshaft 26 (engine speed) and a coolant temperaturesensor for detecting the temperature of the engine coolant. Furthersensors include an accelerator sensor for detecting the acceleratorpedal depression amount and a brake switch for detecting whether or nota brake pedal is depressed. Other sensors include a position sensor fordetecting the valve timing of the intake valve 30, a position sensor fordetecting the valve timing of the exhaust valve 32, and an working anglesensor for detecting the valve working angle of the intake valve 30,which is set by the variable working angle mechanism 46 (i.e., theactuation amount of the variable working angle mechanism 46).

The internal combustion engine 10 includes an electronic control unit(ECU) 50 incorporating a microcomputer or the like. The ECU 50 receivesdetection signals from the various sensors and performs variouscomputations. Based on results of these computations, the ECU 50executes the cooperative control as well as the engine control includingcontrol of the drive of the injector 20 (fuel injection control),control of the actuation of the intake variable valve timing mechanism38, and control of the actuation of the exhaust variable valve timingmechanism 42.

The ECU 50 executes various type of engine control such as idle speedcontrol. In the idle speed control, a target engine speed during idlingof the internal combustion engine 10 (target idle speed Tni) is computedbased on the temperature of the engine coolant. The target idle speedTni is set at a value that minimizes fuel consumption while continuingstable operation of the internal combustion engine 10. The cooperativecontrol and the fuel injection control are then executed such that thetarget idle speed Tni matches the engine speed.

When an advancement failure occurs in the intake variable valve timingmechanism 38, the idle speed control sets the target idle speed Tni at avalue that is higher than when no advancement failure occurs. Suchcontrol is referred to as “idle-up control”.

More specifically, it is determined that an advancement failure hasoccurred when the internal combustion engine 10 is idling if a state inwhich the valve timing of the intake valve 30 is advanced from itstarget by a predetermined amount or more continues for a predeterminedtime or longer. In this case, a lower limit value Gni is set for thetarget idle speed Tni. Further, based on the lower limit value Gni,lower limit guard processing is executed. For example, if the targetidle speed Tni that is computed based on the engine coolant temperatureis lower than the lower limit value Gni, the lower limit value Gni isset as the target idle speed Tni.

Idle-up increases the injected fuel amount and the intake air amount soas to increase the engine speed. This prevents the occurrence ofmisfires that would be caused by an advancement failure in the intakevariable valve timing mechanism 38.

In this embodiment, it is also determined whether a retardation failurehas occurred in the exhaust variable valve timing mechanism 42 when theinternal combustion engine 10 is idling. A retardation failure refers toa state in which the valve timing of the exhaust valve 32 is retardedfrom its normal timing (retarded from a target during variable controlof the valve timing). More specifically, it is determined that aretardation failure has occurred when the internal combustion engine 10is idling if a state in which the valve timing of the exhaust valve 32is retarded from its target for a predetermined time or more continuesfor a predetermined amount or longer.

The ECU 50 makes such a determination of occurrence of an advancementfailure in the intake variable valve timing mechanism 38 and occurrenceof a retardation failure in the exhaust variable valve timing mechanism42.

FIG. 4 shows the valve working angle of the intake valve 30 and thevalve overlap amount. The valve overlap amount refers to the periodduring which the valve working angle of the intake valve 30 overlapswith the valve working angle of the exhaust valve 32.

As shown in FIG. 4, the valve overlap amount decreases as the valveworking angle of the intake valve 30 decreases. Therefore, if anadvancement failure occurs in the intake variable valve timing mechanism38, the internal EGR decreases as the valve working angle of the intakevalve 30 decreases. For this reason, the combustion conditionsdeteriorate less when the valve working angle of the intake valve 30 issmall. Accordingly, misfires are effectively prevented by decreasing thevalve working angle of the intake valve 30 when an advancement failureoccurs even if the increase amount of the target idle speed Tni is smallwhen performing the idle-up control.

In this embodiment, the valve timing of the intake valve 30 is advancedby decreasing the valve working angle of the intake valve 30 since avalve overlap amount is always necessary. Therefore, if the valveworking angle of the intake valve 30 is extremely small, the valvetiming of the intake valve 30 is advanced to a timing that is close tothe limit. Accordingly, in this state, an increase in the valve overlapamount resulting from an advancement failure or deterioration of thecombustion conditions resulting from an advancement failure seldomoccurs.

Accordingly, in this embodiment, the lower limit value Gni of the targetidle speed Tni is set at a lower value when the valve working angle ofthe intake valve 30 is smaller than a predetermined value than when thevalve working angle is larger than the predetermined value.

The procedures for setting the lower limit value Gni (lower limit valuesetting processing) will be described in detail with reference to theflowchart of FIG. 5. The series of procedures shown in the flowchart areexecuted by the ECU 50 in predetermined cycles.

As shown in FIG. 5, it is first determined whether or not certainconditions are satisfied (step S100). The following two conditions mustboth be satisfied.

1) The internal combustion engine 10 is idling. More specifically, theinternal combustion engine 10 is determined to be idling when theaccelerator pedal is not being depressed or the brake pedal is beingdepressed.

2) An advancement failure has occurred in the intake variable valvetiming mechanism 38 or a retardation failure has occurred in the exhaustvariable valve timing mechanism 42.

If the conditions are not satisfied (NO in step S100), the processing istemporarily terminated without proceeding further.

When the conditions are satisfied after repeated execution of theprocessing (YES in step S100), it is then determined whether the timeduring which the precondition has continuously been satisfied is lessthan a predetermined time T (step S102).

If the time is less than the predetermined time T (YES in step S102),the lower limit value Gni is set as described below.

It is first determined whether or not the valve working angle of theintake valve 30 is smaller than a predetermined value An (step S104).

If the valve working angle of the intake valve 30 is equal to or greaterthan the predetermined value An (NO in step S104), the lower limit valueGni is set to a fixed value Gup (step S106).

In this case, it is highly possible that the valve overlap amount hasbeen increased since the valve working angle of the intake valve 30 isrelatively large. This may seriously deteriorate the combustionconditions. Moreover, the internal combustion engine 10 has just startedto idle. Thus, the engine speed may suddenly decrease and temporarilybecome much lower than the target idle speed Tni. In such a state, theengine speed may become excessively low.

Accordingly, the lower limit value Gni is set to the fixed value Gup.The fixed value Gup is determined based on experimental results or thelike so as to prevent excessive decrease in the engine speed even if theengine speed suddenly decreases immediately after the internalcombustion engine 10 starts to idle.

If the valve working angle of the intake valve 30 is smaller than thepredetermined value An (YES in step S104), the increase in the valveoverlap amount will be small even if the valve overlap amount increases.Therefore, deterioration in the combustion conditions is subtle. In sucha case, the processing is temporarily terminated without setting thelower limit value Gni. In other words, the setting of the lower limitvalue Gni and the execution of idle-up control are disabled. Thisprevents an increase in noise level that would be caused by an excessiveincrease in the engine speed and also prevent the fuel efficiency frombeing deteriorated that would be caused by an unnecessary increase inthe fuel injection amount.

When the time during which the conditions are continuously satisfiedbecomes longer than the predetermined time T after repeated execution ofthis processing (NO in step S102), the lower limit value Gni is set inthe following manner.

First, it is determined whether or not the valve working angle of theintake valve 30 is smaller than the predetermined value An (step S108).

If the valve working angle of the intake valve 30 is equal to or greaterthan the predetermined value An (NO in step S108), the lower limit valueGni is determined from a map A based on the valve overlap amount (stepS110).

In this case, it is highly possible that the valve overlap amount hasincreased since the valve working angle of the intake valve 30 is set ata relatively large value. This may seriously deteriorate the combustionconditions. To avoid such a state, the lower limit value Gni is set inaccordance with the valve overlap amount. More specifically, the lowerlimit value Gni is set to a value that prevents misfires from occurringwhile minimizing the increase in fuel consumption after the engine speedis relatively stabilized and does not fluctuate. That is, the lowerlimit value Gni is a value that limits the target idle speed Tni at ahigher speed as the valve overlap amount becomes larger and limits thetarget idle speed Tni at a lower speed than the fixed value Gup.

Referring to FIG. 6, map A is used to determine the lower limit valueGni based on the valve overlap amount. In map A, the relationshipbetween the lower limit value Gni and the valve overlap amount is setbased on experimental results or the like. The valve overlap amount usedin the map computation is obtained based on the valve timing and valveworking angle of the intake valve 30 and the valve timing of the exhaustvalve 32.

If the valve working angle of the intake valve 30 is smaller than thepredetermined value An (YES in step S108), the lower limit value Gni isdetermined from map B based on the valve timing of the exhaust valve 32(step S112).

The reason for using the valve timing of the exhaust valve 32 as aparameter for computing the lower limit value Gni will now be described.

In an internal combustion engine that variably sets the valve timing ofthe exhaust valve, the closing timing of the exhaust valve may be set ata timing that is significantly retarded from the top dead center of thepiston. In such a case, the exhaust valve will be opened for arelatively long period during an intake stroke. Therefore, even if thevalve working angle of the intake valve is small and the valve overlapamount small, exhaust discharged into an exhaust passage is drawn backinto the cylinder. This increases internal EGR. Accordingly, even if thevalve working angle of the intake valve is small, internal EGR maysignificantly increase and cause misfires when the closing timing of theexhaust valve is greatly retarded.

In the internal combustion engine 10 of this embodiment, the internalEGR will not increase to a level that causes misfires. Still, thecombustion conditions may deteriorate. In order to prevent suchdeterioration, the lower limit value Gni is set in accordance with thevalve timing of the exhaust valve 32.

Referring to FIG. 7, map B is used to obtain the lower limit value Gnibased on the valve timing of the exhaust valve 32. Map B, which is setbased on experimental results of the like, shows the relationshipbetween the valve timing of the exhaust valve 32 and the lower limitvalue Gni that effectively prevents deterioration of the combustionconditions while minimizing increase in fuel consumption after theengine speed becomes relatively stable.

The lower limit value Gni determined from the map B is a valuecorresponding to a speed that is much lower than the target idle speedTni. Thus, the lower limit value Gni does not affect the target idlespeed Tni in most of the map B. In other words, the lower limit valueGni is a value at which idle-up is not performed. However, when thevalve timing of the exhaust valve 32 is significantly retarded, thelower limit value Gni that restricts the target idle speed increases asthe valve timing is further retarded, and the idle-up is performed basedon the increased lower limit value Gni. The lower limit value Gniobtained in this case is a value corresponding to a speed that is lowerthan the lower limit value Gni when the valve working angle of theintake valve 30 is smaller than the predetermined value An.

After the lower limit value Gni is obtained in this manner, theprocessing is temporarily terminated.

This embodiment has the advantages described below.

(1) When an advancement failure occurs in the intake variable valvetiming mechanism 38, the lower limit value Gni for limiting the targetidle speed Tni is set to be lower when the valve working angle of theintake valve 30 is low compared to when the valve working angle of theintake valve 30 is high. Accordingly, for combustion conditions thatdiffer depending on settings of the valve working angle of the intakevalve 30, the lower limit value Gni and the increase amount of thetarget idle speed Tni is set at an appropriate value in accordance withthe effect on the combustion conditions. This prevents the occurrence ofmisfires. Further, an increase in noise level that would be caused byexcessive increase in the engine speed is prevented. The fuel efficiencyis also prevented from being lowered such as when the fuel injectionamount is excessive. As a result, it is possible to prevent theoccurrence of misfire more efficiently and effectively during occurrenceof an advancement failure in the intake variable valve timing mechanism38.

(2) The execution of idle-up control is disabled when the valve workingangle of the intake valve 30 is smaller than the predetermined value An.Therefore, the target idle speed Tni is not increased when the valveworking angle is smaller than the predetermined value An, that is, whenthe increase in the degree of internal EGR due to occurrence of anadvancement failure is relatively small and the possibility ofoccurrence of misfire is relatively low. This prevents the engine speedand injected fuel amount from being unnecessarily increased. As aresult, the occurrence of misfires during occurrence of an advancementfailure is prevented more efficiently and effectively.

(3) The lower limit value Gni is determined based on the valve overlapamount when the valve working angle of the intake valve 30 is largerthan the predetermined value An after certain conditions have beencontinuously satisfied for the predetermined time T or longer, that is,when the combustion conditions may be deteriorated by the increase inthe valve overlap amount. The lower limit value Gni is obtained based onthe valve timing of the exhaust valve 32, when the valve working angleof the intake valve 30 is smaller than the predetermined value An, thatis, when the combustion conditions may be deteriorated due to theclosing timing of the exhaust valve 32 being greatly retarded.Accordingly, the lower limit value Gni and the target idle speed Tni areset using parameters that are currently most appropriate for the valveworking angle of the intake valve 30. This prevents the occurrence ofmisfires during occurrence of an advancement failure further efficientlyand effectively.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Particularly, it should beunderstood that the present invention may be embodied in the followingforms.

In the preferred embodiment, the lower limit value Gni is determinedbased on the valve timing of the exhaust valve 32 when the valve workingangle of the intake valve 30 is smaller than the predetermined value Anafter the certain conditions have been continuously satisfied for thepredetermined time T or longer. The valve timing used for determiningthe lower limit value Gni may be, for example, the opening timing of theexhaust valve 32, the timing when the displacement of the exhaust valve32 becomes maximum, a timing indicative of the closing timing of theexhaust valve 32, or the closing timing of the exhaust valve 32 itself.

When the idle speed controller of the preferred embodiment is applied toa vehicle internal combustion engine having a transmission, a gearshiftposition of the transmission may also be employed as one of theparameters for determining the lower limit value Gni that is used in theprocessing of steps S110 and S112.

If the internal combustion engine 10 is capable of preventing theexcessive decrease in the engine speed immediately after idling starts,the processing steps of S102 to S106 may be omitted as shown in theflowchart of FIG. 8.

In the preferred embodiment, the setting of the lower limit value Gni ischanged when the valve working angle of the intake valve 30 is smallerthan the predetermined value An and when the valve working angle of theintake period is equal to or greater than the predetermined value An.Instead, the setting of the lower limit value Gni may be changed threeor more times in accordance with the valve working angle of the intakevalve 30. Also, instead of changing the setting for a number of times,the lower limit value Gni may be set variably in accordance with thevalve working angle of the intake valve 30 by determining the lowerlimit value Gni based on the valve working angle of the intake valve 30.It is only necessary that the lower limit value Gni for limiting thetarget idle speed Tni be lower when the valve working angle of theintake valve 30 is smaller than the predetermined value An than when thevalve working angle is larger than the predetermined value An.

The idle speed controller of the preferred embodiment is also applicableto an internal combustion engine that is not provided with an exhaustvariable valve timing mechanism 42. When the idle speed controller isapplied to such an internal combustion engine, the step S112 may beomitted from the processing procedures for setting the lower limitvalue.

The present invention is not limited to a controller that executesidle-up control by setting the lower limit value for the target idlespeed when performing the idle speed control. The present invention isalso applicable to an idle speed controller that executes idle-upcontrol by changing the method for computing the target idle speed or bycomputing a correction amount for changing the target idle speed. Whenthe present invention is applied to such a controller, idle-up controlmay be performed during occurrence of an advancement failure by settingthe increase amount in the target idle speed smaller when the valveworking angle of the intake valve is smaller than a predetermined valuethan when the valve working angle is larger than the predeterminedvalue. Alternatively, the idle-up control during occurrence of anadvancement failure may be disabled when the valve working angle of theintake valve is smaller than the predetermined value.

In addition to an internal combustion engine in which the valve workingangle of an intake valve is variably set in accordance with whether ornot the engine has been warmed, the present invention may also beapplied to an internal combustion engine in which the valve workingangle of an intake valve is variably set in accordance with depositsadhered to the intake valve or its valve seat, in accordance with thestate of use of electrical equipment such as headlights, or inaccordance with the state of operation of an engine accessory such as anair compressor or an alternator.

In addition to an internal combustion engine having a variable workingangle mechanism for variably setting the valve working angle of anintake valve, the present invention may also be applied to an internalcombustion engine having a variable working angle mechanism for variablysetting the valve working angle of an exhaust valve. In this case,idle-up control may be executed during occurrence of an advancementfailure by setting the increase amount in the target idle speed smallerwhen the valve working angle of the exhaust valve is smaller than apredetermined value than when the valve working angle is larger than thepredetermined value. Alternatively, idle-up control may be disabledduring occurrence of an advancement failure when the valve working angleof the exhaust valve is smaller than the predetermined value.

The present invention may also be applied to an internal combustionengine having an exhaust variable valve timing mechanism and a variableworking angle mechanism, which variably sets a valve working angle of anintake valve. The present invention may also be applied to an internalcombustion engine having an exhaust variable valve timing mechanism anda variable working angle mechanism, which variably sets a valve workingangle of an exhaust valve.

When a retardation failure occurs in the exhaust variable valve timingmechanism, the valve overlap amount of the intake valve and the exhaustvalve becomes smaller and the internal EGR becomes less as the valveworking angle of the intake valve (or the exhaust valve) is decreased bythe variable working angle mechanism. Accordingly, the deterioration ofthe combustion conditions becomes lower as the valve working angle issmaller. Therefore, the deterioration in the combustion conditionsbecomes lower as the valve working angle becomes smaller. Thus, theoccurrence of misfires is effectively prevented even if the increaseamount in the target idle speed is set to a small value in the idle-upcontrol during occurrence of a retardation failure.

Therefore, when idle-up control is performed during occurrence of aretardation failure in the internal combustion engine, the increaseamount in the target idle speed may be set to a smaller value when thevalve working angle set by the variable working angle mechanism issmaller than a predetermined value than when the valve working angle islarger than the predetermined value. Thus, the increase amount of thetarget idle speed is appropriately set in accordance with the valveworking angle and the effect on the combustion conditions of the engine.As a result, it is possible to prevent the occurrence of misfires duringoccurrence of a retardation failure more efficiently and effectively,while suppressing excessive increase in the engine speed.

Further, in the internal combustion engine, idle-up control duringoccurrence of a retardation failure may be disabled when the valveworking angle set by the variable working angle mechanism is smallerthan a predetermined value. In this case, the increase in the targetidle speed may be disabled when the valve working angle is smaller thanthe predetermined value, that is, when increase in the internal EGR dueto the occurrence of a retardation failure is relatively low and thepossibility of occurrence of misfires is low. Accordingly, it ispossible to suppress the excessive increase in the engine speed andprevent the occurrence of misfires during occurrence of a retardationfailure further efficiently and effectively.

The present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

1. An idle speed controller for executing idle speed control for aninternal combustion engine including an intake valve and an exhaustvalve, the idle speed controller comprising: either one of an intakevariable working angle mechanism for variably setting an working angleof the intake valve and an exhaust variable working angle mechanism forvariably setting an working angle of the exhaust valve; a variable valvetiming mechanism for variably setting valve timing for the intake valve;and an electronic control unit for executing idle speed control; whereinwhen the internal combustion engine is idling and an advancement failureof the variable valve timing mechanism occurs thereby advancing thevalve timing of the intake valve from an intended timing, the electroniccontrol unit performs idle-up during execution of the idle speed controlto increase a target idle speed; and wherein when the working angle setby either one of the intake and exhaust variable working angle mechanismis less than a predetermined value, the electronic control unit changesan increase amount of the target idle speed to a lower value if anadvancement failure occurs when performing the idle-up as compared towhen the working angle is greater than the predetermined value.
 2. Theidle speed controller according to claim 1, wherein: the internalcombustion engine further includes a variable exhaust valve closingtiming mechanism for varying closing timing for the exhaust valve; andthe electronic control unit computes the target idle speed based on anoverlap amount of valve opening periods of the intake valve and theexhaust valve when the working angle set by either one of the intake andexhaust variable working angle mechanisms is greater than thepredetermined value, and the electronic control unit computes the targetidle speed based on the closing timing of the exhaust valve when theworking angle is less than the predetermined value.
 3. An idle speedcontroller for executing idle speed control for an internal combustionengine including an intake valve and an exhaust valve, the idle speedcontroller comprising: either one of an intake variable working anglemechanism for variably setting an working angle of the intake valve andan exhaust variable working angle mechanism for variably setting anworking angle of the exhaust valve; a variable valve timing mechanismfor variably setting valve timing of the intake valve; and an electroniccontrol unit for executing idle speed control; wherein when the internalcombustion engine is idling and an advancement failure of the variablevalve timing mechanism occurs thereby advancing the valve timing of theintake valve from an intended timing, the electronic control unitperforms idle-up during execution of the idle speed control to increasea target idle speed; and wherein when the working angle set by eitherone of the intake and exhaust variable working angle mechanism is lessthan a predetermined value, the electronic control unit prohibits theidle-up if an advancement failure occurs.
 4. The idle speed controlleraccording to claim 3, wherein: the internal combustion engine furtherincludes a variable exhaust valve closing timing mechanism for varyingclosing timing of the exhaust valve; and the electronic control unitcomputes the target idle speed based on an overlap amount of valveopening periods of the intake valve and the exhaust valve when theworking angle set by either one of the intake and exhaust variableworking angle mechanisms is greater than the predetermined value, andthe electronic control unit computes the target idle speed based on theclosing timing of the exhaust valve when the working angle is less thanthe predetermined value.
 5. The idle speed controller according to claim4, wherein the electronic control unit computes the target idle speedwhen the engine is idling if the valve timing of the intake valve hascontinuously been advanced from its control target position by apredetermined amount or more for a predetermined time or more.
 6. Theidle speed controller according to claim 5, wherein the electroniccontrol unit determines whether or not to perform idle-up based on theworking angle when the engine is idling if a state in which the valvetiming of the intake valve is advanced from its control target positionby a predetermined amount or more does not continue for a predeterminedtime or more.
 7. The idle speed controller according to claim 6, whereinthe electronic control unit performs idle-up by setting the target idlespeed to a fixed value when the working angle is greater than thepredetermined value, and the electronic control unit prohibits idle-upwhen the working angle is less than the predetermined value.
 8. The idlespeed controller according to claim 5, wherein the electronic controlunit computes the target idle speed based on coolant temperature of theengine.
 9. The idle speed controller according to claim 6, wherein theelectronic control unit computes the target idle speed based on coolanttemperature of the engine.
 10. The idle speed controller according toclaim 8, wherein the electronic control unit sets a lower limit valuefor the target idle speed when an advancement failure occurs, andperforms the idle-up by setting the target idle speed based on the lowerlimit value if the computed target idle speed is lower than the lowerlimit value.
 11. The idle speed controller according to claim 10,wherein the lower limit value for the target idle speed is increased inaccordance with the overlap amount of the valve opening periods of theintake valve and the exhaust valve.
 12. The idle speed controlleraccording to claim 11, wherein the lower limit value for the target idlespeed, set in accordance with the overlap amount, is set to be lowerthan the fixed value to which the target idle speed is set if the valvetiming of the intake valve does not continues to be advanced from itscontrol target position by a predetermined amount or more for apredetermined time or more when the engine is idling.
 13. The idle speedcontroller according to claim 11, wherein the electronic control unitexecutes fuel injection amount control and an air amount control so thatthe engine speed becomes equal to the target idle speed.
 14. The idlespeed controller according to claim 11, wherein the air amount controlincludes controlling an open amount of a throttle valve and controllingactuation of the variable working angle mechanism.
 15. An idle speedcontroller for executing idle speed control for an internal combustionengine including an intake valve and an exhaust valve, the idle speedcontroller comprising: either one of an intake variable working anglemechanism for variably setting an working angle of the intake valve andan exhaust variable working angle mechanism for variably setting anworking angle of the exhaust valve; a variable valve timing mechanismfor variably setting valve timing of the intake valve; and an electroniccontrol unit for executing idle speed control; wherein when the internalcombustion engine is idling and a retardation failure of the variablevalve timing mechanism occurs thereby retarding the valve timing of theintake valve from an intended timing, the electronic control unitperforms idle-up during execution of the idle speed control to increasea target idle speed; and wherein when the working angle set by eitherone of the intake and exhaust variable working angle mechanism is lessthan a predetermined value, the electronic control unit changes anincrease amount of the target idle speed to a lower value if aretardation failure occurs when performing the idle-up as compared towhen the working angle is greater than the predetermined value.
 16. Anidle speed controller for executing idle speed control for an internalcombustion engine including an intake valve and an exhaust valve, theidle speed controller comprising: either one of an intake variableworking angle mechanism for variably setting an working angle of theintake valve and an exhaust variable working angle mechanism forvariably setting an working angle of the exhaust valve; a variable valvetiming mechanism for variably setting valve timing of the intake valve;and an electronic control unit for executing idle speed control; whereinwhen the internal combustion engine is idling and a retardation failureof the variable valve timing mechanism occurs thereby retarding thevalve timing of the intake valve from an intended timing, the electroniccontrol unit performs idle-up during execution of the idle speed controlto increase a target idle speed; and wherein when the working angle setby either one of the intake and exhaust variable working angle mechanismis less than a predetermined value, the electronic control unitprohibits the idle-up if an advancement failure occurs.